Electronic control systems for internal combustion engines frequently utilize a rotatable valve disposed in an engine air induction passage to regulate the air flow through the passage. The valve can be a throttle valve which is positioned by an operator to control air flow to the engine. Air flow to the engine is controlled by varying the flow area around the valve.
The flow area around the valve is geometrically related to the angular position of the valve. In many valve assemblies, proportional changes in the flow area (i.e., the ratio of the flow area change to the flow area) caused by rotation of the valve through a small arc are greater when the valve is near its minimum flow position (i.e., the valve position wherein the air flow area is minimum), as compared to when it is away form the minimum flow position. This is due in part to the flow area being smaller when the valve is near the minimum flow position as compared to when it is away from the minimum flow position. As a result, deviations between the actual and desired valve positions have a more pronounced effect on the flow area when the valve is near the minimum flow position as compared to when it is away from the minimum flow position. Therefore, assuming that the proportional change between the actual and desired flow areas for a given valve position is limited, the permissible deviation between the actual and desired valve positions when the valve is near the minimum flow position is less than the permissible deviation when the valve is away from the minimum flow position. Thus, while it may be possible to limit deviations between the actual and desired valve positions to an acceptable amount when the valve is away from the minimum flow position, it can be difficult to limit such deviations when the valve is near the minimum flow position. If the deviation of the actual valve position from the desired valve position exceeds the permissible amount when the valve is near the minimum flow position, then control of the engine when the flow area is low can be difficult.
Many valve assemblies used to control air flow to vehicle engines return the valve to the minimum flow position when the mechanism which controls the valve position is deactivated. The flow are when the valve is in this position does not typically allow sufficient air flow around the valve to operate the engine in a fast idle condition that allows limp home operation. p Moreover, in some valve assemblies having a valve member supported on a shaft, an axial load along the shaft can cause the valve member to scrape the walls of the valve bore. The wear which can result increases the flow area. This allows additional air flow around the valve member thereby altering the original calibration of the valve assembly. Substantial increases in the minimum flow are can result. This can further decrease control of the air flow to the engine when the flow area is low. Also, scraping by the valve member on the wall of the valve bore can generate friction, and increase deviations between the actual and desired valve positions. Mechanisms to control endplay of a shaft in a valve assembly to minimize such wear are known, but many are difficult to assemble to the valve body.